The invention relates to optical apparatus and methods for switching optical signals, more especially to apparatus and methods for switching optical signals by a gating process.
In optical telecommunications, three separate wavelength ranges (xe2x80x9cwindowsxe2x80x9d) have been identified for transmitting optical signals, these windows corresponding to spectral regions where the attenuation and dispersion properties of light through silica optical fibers are favorable. The first window is centered at 0.9 microns. The second window is centered at 1.3 microns. The third window is centered at 1.55 microns. It is the second and third windows which are perhaps the most important commercially.
Optical switches are a fundamental component for optical communications networks. Optical switches can be controlled electrically (opto-electronic switching) or optically (all-optical switching). Opto-electronic switching is limited in speed by the bandwidth of electronic drive circuits to about 10 Gb/s, whereas all-optical switching can be much faster, e.g. 100 Gb/s.
An all-optical switch based on an erbium-doped fiber has been proposed [3,4]. Usefully, this optical switch is operable in the typical wavelength range of amplification of the Er3+ions (1530-1560 nm) used in erbium-doped fiber amplifiers (EDFAs). However, the rise time of the switching in this erbium-doped device is rather slow, being in the tens of microseconds range. The switch-on time of amplification is intrinsically limited to 7 xcexcs or more due to the long fluorescence lifetime of erbium ions in glass. As a result, rapid optical switching, e.g. of packets or trains of pulses, as would be desirable for a packet switching network such as an ATM (Asynchronous Transfer Mode) network, is not possible. The opening time of such switches is simply too long.
It is therefore an aim of the invention to provide an optical switch capable of switching at high speed, in particular in the second and/or third telecommunications windows.
According to a first aspect of the invention there is provided a method of gating an optical signal comprising:
(a) inputting an optical signal to be gated into a gain medium comprising a transition metal dopant;
(b) applying a first control signal to the gain medium to populate an excited state of the transition metal dopant, thereby to induce amplification of the optical signal by stimulated emission; and
(c) applying a second control signal to the gain medium to depopulate the excited state of the transition metal dopant, thereby to suppress the amplification of the optical signal.
It has been discovered that transition metal doped materials, such as Cr4+or V3+doped crystals can be used as the basis for optical switching. More specifically, it has been realized that transition metal dopants can provide amplification that can be switched on and off very rapidly in comparison to Er3+rare-earth dopant based optical switching where the switch-on is inherently slow.
With transition metal dopants, the rise-time of the amplification process is limited only by: (a) the duration of the first control signal which is operable to provide an xe2x80x9conxe2x80x9d pulse; and (b) the time for the excited electrons to relax to the lowest level of the excited state, this time generally being short for transition metal dopants (in contrast to the rare earth dopant Er3+which has a much slower relaxation time). For example, the relaxation time is known to be only a few picoseconds in the case of the 3T2 state of Cr4+in a YAG (Yttrium Aluminum Garnet: Y3Al5O12) host crystal [1] and it is this picosecond relaxation time that will define the ultimate speed limit of Cr4+:YAG based embodiments of the invention. Importantly, the short rise-time allows a transition metal doped switch to be used to gate trains of high frequency signal pulses, such as occur in an optical packet switching network, for example.
To the inventors"" knowledge, there is no description or suggestion in the prior art that transition metal doped crystals could be used for optical switching, although it is known to the inventors that Cr4+:YAG has been used as laser rod material [1, 5, 6, 11].
It has also been realized that transition metal dopants have a further significant advantage over rare earth dopants in that, with a transition metal, the basic atomic electronic levels are broadened out into bands by the presence of the host crystal to a much greater extent than for rare earth dopants which are relatively unaffected by their host crystal. As a result, a transition metal dopant based device can provide a relatively broad wavelength range for amplification. In the specific example of Cr4+:YAG, amplification occurs over a highly desirable broad band extending between about 1.3-1.6 microns, spanning both the second and third telecommunications windows.
In the first aspect of the invention, the first control signal may be provided by a first light source having an output at a first wavelength at which there is a first allowed optical transition of the transition metal dopant into the excited state. The second control signal may be provided by a second light source having an output at a second wavelength at which there is a second allowed optical transition of the transition metal dopant out of the excited state. The optical signal to be gated may be provided by a third light source of any wavelength within the amplification band of the gain medium. In a preferred embodiment of the invention, the gain medium provides amplification over a wavelength range encompassing one or both of the second and third telecommunications windows. The first and second control signals are preferably optical, although electrical or other excitation may be possible with some materials.
According to a second aspect of the invention there is provided an apparatus for gating an optical signal comprising:
(a) an input stage arranged to receive and combine first and second optical control signals of first and second wavelengths, and an optical signal to be gated of a third wavelength;
(b) a gain medium arranged to receive from the input stage the first and second optical control signals and the optical signal to be gated, the gain medium comprising a transition metal dopant that has an excited state populatable by the first optical control signal and depopulatable by the second optical control signal to allow selective amplification of the optical signal to be gated by stimulated emission from the excited state responsive to the first and second optical control signals; and
(c) an output stage for transmitting the optical signal after gating in the gain medium.
The apparatus of the second aspect of the invention may further comprise: a first light source for generating the first optical control signal at a first wavelength for populating the excited state of the transition metal dopant so as to provide amplification of the optical signal at the third wavelength; and/or a second light source for generating the second optical control signal at a second wavelength for depopulating the excited state of the transition metal dopant so as to suppress amplification of the optical signal at the third wavelength. In some embodiments, one or both of the first and second light sources can be integrated with the gain medium to form a single component. In other embodiments, one of both of the first and second light sources may be provided separately from the gain medium, e.g. remotely to allow remote operation of the active part of the apparatus.
The apparatus may be a free-space apparatus, an optical fiber based apparatus or a planar waveguide based apparatus, as desired. In addition, the apparatus may be a hybrid mixture of free-space, fiber and/or solid state waveguide components. In one embodiment, at least one of the gain medium, the input stage and the output stage are located in an optical fiber. In another embodiment, at least one of the gain medium, the input stage and the output stage is located in a planar waveguide. One or more of the light sources may also be integrated with these components in an integrated fiber or planar waveguide component, e.g. optical fiber lasers or solid state lasers.
In embodiments of the invention, it is useful to provide a filter for filtering out the first and second control signals from the optical signal after passing through the gain medium so that the output from the gating device is substantially free of control ignal components.
In the best mode of the invention, the transition metal dopant is Cr4+and the gain medium comprises YAG as a host crystal for the transition metal dopant. Other transition metal dopants such as V4+may also be suitable. Moreover, for any given transition metal dopant, different host crystals may be selected to change the energies of the same quantum transitions, thereby allowing a degree of selection of the wavelength range over which amplification occurs. Specifically, further embodiments are contemplated using Cr4+in hosts other than YAG, for example in other garnets. The design freedom provided by selection of the host for a given transition metal dopant is not available with rare-earth dopants, such as the Er3+used in the prior art [3, 4], since the transition energies of rare-earth dopants are substantially unaffected by changes of host crystal, owing to the electronic shell configuration of the rare earths.